Several gantry designs for proton and carbon beam therapy exist and are commercially provided by multiple manufacturers. The existing designs are all based upon dipole-type bending magnets. Because of the stray magnetic field, either a large amount of external ferromagnetic shielding is required (such as iron), or a second set of coils with reversed field are needed to reduce or eliminate the fringe magnetic field from the dipole. In either case, the weight of the magnets is high. Means to reduce the weight and size of the system are desirable. In addition, some modalities of particle beam therapy require beam scanning. The goal is for the Bragg peak to be scanned in all three dimensions: ordinate and coordinate (within the treatment plane at fixed beam energy) and in depth (varying the location of the treatment plane within the body). Depth variation requires the ability to vary the energy of the beam. Magnets that can compensate for the varying beam energy are being investigated. One solution that has been proposed is achromatic magnets. These magnets have two conventional dipoles separated by one or more quadrupole (focusing) magnets. The quadrupole magnets compensate for dispersion and shift in beam direction because of the variation in beam energy. The variation in beam energy is limited before the amplitude of the magnetic field needs to be adjusted.
In addition, because of the desire to be able to scan the beam, a large aperture is required, at least for the final bending magnet. The scanning beam may be upstream from the last bending magnet. To prevent interception the beam, the aperture at the exit of the last bending magnet needs to be fairly large.
The use of toroidal magnets for beam physics application is well known. In particular, one of the largest, most powerful bending magnets, the ATLAS Barrel Torus, is a toroidal magnet. In the ATLAS magnet design, the magnetic field actually has substantial radial variation and has relatively low aspect ratio, unfavorably affecting the optics of the beam. Large coil aspect ratio is desirable, as will be described below, to minimize the size and weight of the toroidal bending magnet. The toroidal magnet has the advantage that there is no need, or minimal need, for magnetic shielding. Thus, the iron required in the case of simple dipoles, or the second coil needed to cancel the far fields, is not needed, substantially decreasing the weight of the system. However, this type of bending magnets has not been considered in hadron beam therapies.
It would be beneficial if toroidal magnets could be modified for use with hadron beam therapies.